1. Field of the Invention
The present invention relates to an air fuel ratio control apparatus for an internal combustion engine installed on a vehicle or the like. In particular, the invention relates to an air fuel ratio control apparatus for an internal combustion engine provided with an air fuel ratio feedback control section for oscillating the air fuel ratio of a mixture supplied to the internal combustion engine in rich and lean directions in a periodic manner.
2. Description of the Related Art
In general, a three-way catalyst (hereinafter referred to simply as a “catalyst”) for purifying harmful components HC, CO, NOx in an exhaust gas at the same time is installed in the exhaust passage of an internal combustion engine, and in this kind of catalyst, the purification rate of the harmful components HC, CO, NOx becomes high in the vicinity of the stoichiometric air fuel ratio. Accordingly, in the air fuel ratio control apparatus for an internal combustion engine, an oxygen sensor is generally arranged at a location upstream of the catalyst, the air fuel ratio, and the air fuel ratio of a mixture is controlled in a feedback manner by adjusting the amount of injection fuel to a value in the vicinity of the stoichiometric air fuel ratio.
In addition, an oxygen occlusion capability, acting like filter processing, is added to the catalyst, so that a temporary variation of an upstream air fuel ratio (corresponding to an output value of an upstream oxygen sensor) from the stoichiometric air fuel ratio is absorbed. That is, the catalyst takes in the oxygen contained in the exhaust gas when the upstream air fuel ratio (hereinafter referred to as an “upstream A/F”) is leaner than the stoichiometric air fuel ratio, whereas it releases the oxygen accumulated in the catalyst when the upstream A/F is richer than the stoichiometric air fuel ratio. Accordingly, the variation of the upstream A/F is filter processed in the catalyst, thus resulting in an air fuel ratio downstream of the catalyst.
Also, a maximum value of the amount of oxygen occlusion of the catalyst is decided by an amount of a material having an oxygen occlusion capability attached upon production of the catalyst, and the variation of the upstream A/F can not be absorbed any more when the amount of oxygen occlusion reaches a maximum amount of oxygen occlusion or a minimum amount of oxygen occlusion (=0) of the catalyst, so the air fuel ratio in the catalyst deviates from the stoichiometric air fuel ratio to decrease the purification ability of the catalyst. At this time, the air fuel ratio downstream of the catalyst deviates greatly from the stoichiometric air fuel ratio, so it is possible to detect that the amount of oxygen occlusion in the catalyst has reached the maximum value or minimum value (=0).
Further, the catalyst, being exposed to the exhaust gas of a high temperature, is designed such that the purification function of the catalyst is not rapidly reduced in certain conditions of use which can be generally considered in the internal combustion engine for a vehicle. However, the oxygen occlusion capability of the catalyst might remarkably be decreased during the use thereof because of some causes (e.g., in case of a misfire), and in addition, the oxygen occlusion capability is decreased gradually due to aging even when the travel distance of the vehicle reaches tens of thousands of kilometers for example.
Also, as is clear from timing charts of FIG. 34 and FIG. 35 which show the individual behaviors of the catalyst at the time of a normal operation thereof and at the time of a degraded operation thereof, respectively, it is known that as the catalyst is degraded or deteriorated to reduce the maximum amount of oxygen occlusion OSC to a certain value or below, the variation of the output value V1 (upstream A/F) of the upstream oxygen sensor becomes unable to be absorbed by the catalyst, so the variation of the output value V2 (the downstream A/F) of the downstream oxygen sensor increases.
Accordingly, in the past, there has been proposed an air fuel ratio control apparatus for an internal combustion engine that diagnoses the degradation of the catalyst by comparing the variation of an output value V1 of an upstream oxygen sensor and the variation of an output value V2 of a downstream oxygen sensor (see, for example, a first patent document: Japanese patent application publication No. H6-39932).
In addition, in recent years, exhaust emission control is strengthened from enhanced consideration to the earth environment, and hence it is requested to detect much smaller degradation of a catalyst (a decrease in the maximum amount of oxygen occlusion). Also, the thermal resistance of materials having an oxygen occlusion capability is being improved every year, so it is becoming possible to increase an amount of addition of such a material to a catalyst, and a maximum amount of oxygen occlusion of the catalyst for which the detection of degradation is necessary is increasing.
Accordingly, there has also been proposed an air fuel ratio control apparatus for an internal combustion engine in which, as shown in a timing chart of FIG. 36, by increasing the period and oscillation width of oscillation of an air fuel ratio to a lean direction and to a lean direction of an output value V1 (upstream A/F) of an upstream oxygen sensor, a maximum amount of oscillation width OSCmax of an amount of oxygen occlusion OSC of a catalyst is increased so that a slight degradation of the catalyst can be detected (see, for example, a second patent document: Japanese patent application laid-open No. H6-26330, and a third patent document: Japanese patent application laid-open No. H7-259600).
In conventional air fuel ratio control apparatuses for an internal combustion engine, according to a method of increasing the period or oscillation width of the air fuel ratio oscillation, for example, as in the above-mentioned first through third patent documents, there has been a problem that a degradation in air fuel ratio feedback performance with respect to external disturbances and an increase in variation of the output torque of the internal combustion engine might be caused, thus reducing marketability.
On the other hand, it can be considered that an amplitude or oscillation width of an amount of oxygen occlusion of a catalyst is increased by making an average value of an oscillating air fuel ratio oscillate in rich and lean directions in a periodic manner without changing the period and oscillation width of the air fuel ratio oscillation to the rich and lean directions of an upstream A/F to any great extent. However, in case where the average air fuel ratio is made to oscillate in this manner, the amount of oxygen occlusion is made to oscillate by means of oscillation processing of the average air fuel ratio at a period longer than the period of variation of the upstream air fuel ratio, as a result of which variation in a downstream air fuel ratio is generated without correlation to the period of variation of the air fuel ratio. Accordingly, there arises a problem that in case where the degradation of the catalyst is diagnosed based on the correlation between the upstream air fuel ratio variation and the downstream air fuel ratio variation, as in the above-mentioned conventional apparatuses, diagnostic accuracy will be reduced.